Problem: In history class, the girl to boy ratio is $8$ to $2$. If there are $40$ girls, how many boys are there?
Solution: A ratio of $8$ girls to $2$ boys means that there is a group of $8$ girls for every group of $2$ boys. If there are $40$ girls, then there are $5$ groups of $8$ girls each. Because we know the ratio is $8:2$ , there must be $5$ groups of $2$ boys each. There is a total of $10$ boys in history class.